// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
// https://developers.google.com/protocol-buffers/
//
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package com.google.protobuf;

import java.io.IOException;

/**
 * LazyFieldLite encapsulates the logic of lazily parsing message fields. It stores the message in a
 * ByteString initially and then parses it on-demand.
 *
 * <p>LazyFieldLite is thread-compatible: concurrent reads are safe once the proto that this
 * LazyFieldLite is a part of is no longer being mutated by its Builder. However, explicit
 * synchronization is needed under read/write situations.
 *
 * <p>When a LazyFieldLite is used in the context of a MessageLite object, its behavior is
 * considered to be immutable and none of the setter methods in its API are expected to be invoked.
 * All of the getters are expected to be thread-safe. When used in the context of a
 * MessageLite.Builder, setters can be invoked, but there is no guarantee of thread safety.
 *
 * <p>TODO(yatin,dweis): Consider splitting this class's functionality and put the mutable methods
 * into a separate builder class to allow us to give stronger compile-time guarantees.
 *
 * <p>This class is internal implementation detail of the protobuf library, so you don't need to use
 * it directly.
 *
 * @author xiangl@google.com (Xiang Li)
 */
public class LazyFieldLite {
    private static final ExtensionRegistryLite EMPTY_REGISTRY =
            ExtensionRegistryLite.getEmptyRegistry();

    /*
     * The value associated with the LazyFieldLite object is stored in one or more of the following
     * three fields (delayedBytes, value, memoizedBytes). They should together be interpreted as
     * follows.
     *
     * 1) delayedBytes can be non-null, while value and memoizedBytes is null. The object will be in
     * this state while the value for the object has not yet been parsed.
     *
     * 2) Both delayedBytes and value are non-null. The object transitions to this state as soon as
     * some caller needs to access the value (by invoking getValue()).
     *
     * 3) memoizedBytes is merely an optimization for calls to LazyFieldLite.toByteString() to avoid
     * recomputing the ByteString representation on each call. Instead, when the value is parsed from
     * delayedBytes, we will also assign the contents of delayedBytes to memoizedBytes (since that is
     * the ByteString representation of value).
     *
     * 4) Finally, if the LazyFieldLite was created directly with a parsed MessageLite value, then
     * delayedBytes will be null, and memoizedBytes will be initialized only upon the first call to
     * LazyFieldLite.toByteString().
     *
     * <p>Given the above conditions, any caller that needs a serialized representation of this object
     * must first check if the memoizedBytes or delayedBytes ByteString is non-null and use it
     * directly; if both of those are null, it can look at the parsed value field. Similarly, any
     * caller that needs a parsed value must first check if the value field is already non-null, if
     * not it must parse the value from delayedBytes.
     */

    /**
     * A delayed-parsed version of the contents of this field. When this field is non-null, then the
     * "value" field is allowed to be null until the time that the value needs to be read.
     *
     * <p>When delayedBytes is non-null then {@code extensionRegistry} is required to also be
     * non-null. {@code value} and {@code memoizedBytes} will be initialized lazily.
     */
    private ByteString delayedBytes;

    /**
     * An {@code ExtensionRegistryLite} for parsing bytes. It is non-null on a best-effort basis. It
     * is only guaranteed to be non-null if this message was initialized using bytes and an {@code
     * ExtensionRegistry}. If it directly had a value set then it will be null, unless it has been
     * merged with another {@code LazyFieldLite} that had an {@code ExtensionRegistry}.
     */
    private ExtensionRegistryLite extensionRegistry;

    /**
     * The parsed value. When this is null and a caller needs access to the MessageLite value, then
     * {@code delayedBytes} will be parsed lazily at that time.
     */
    protected volatile MessageLite value;

    /**
     * The memoized bytes for {@code value}. This is an optimization for the toByteString() method to
     * not have to recompute its return-value on each invocation. TODO(yatin): Figure out whether this
     * optimization is actually necessary.
     */
    private volatile ByteString memoizedBytes;

    /**
     * Constructs a LazyFieldLite with bytes that will be parsed lazily.
     */
    public LazyFieldLite(ExtensionRegistryLite extensionRegistry, ByteString bytes) {
        checkArguments(extensionRegistry, bytes);
        this.extensionRegistry = extensionRegistry;
        this.delayedBytes = bytes;
    }

    /**
     * Constructs a LazyFieldLite with no contents, and no ability to parse extensions.
     */
    public LazyFieldLite() {
    }

    /**
     * Constructs a LazyFieldLite instance with a value. The LazyFieldLite may not be able to parse
     * the extensions in the value as it has no ExtensionRegistry.
     */
    public static LazyFieldLite fromValue(MessageLite value) {
        LazyFieldLite lf = new LazyFieldLite();
        lf.setValue(value);
        return lf;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }

        if (!(o instanceof LazyFieldLite)) {
            return false;
        }

        LazyFieldLite other = (LazyFieldLite) o;

        // Lazy fields do not work well with equals... If both are delayedBytes, we do not have a
        // mechanism to deserialize them so we rely on bytes equality. Otherwise we coerce into an
        // actual message (if necessary) and call equals on the message itself. This implies that two
        // messages can by unequal but then be turned equal simply be invoking a getter on a lazy field.
        MessageLite value1 = value;
        MessageLite value2 = other.value;
        if (value1 == null && value2 == null) {
            return toByteString().equals(other.toByteString());
        } else if (value1 != null && value2 != null) {
            return value1.equals(value2);
        } else if (value1 != null) {
            return value1.equals(other.getValue(value1.getDefaultInstanceForType()));
        } else {
            return getValue(value2.getDefaultInstanceForType()).equals(value2);
        }
    }

    @Override
    public int hashCode() {
        // We can't provide a memoizable hash code for lazy fields. The byte strings may have different
        // hash codes but evaluate to equivalent messages. And we have no facility for constructing
        // a message here if we were not already holding a value.
        return 1;
    }

    /**
     * Determines whether this LazyFieldLite instance represents the default instance of this type.
     */
    public boolean containsDefaultInstance() {
        return memoizedBytes == ByteString.EMPTY
                || value == null && (delayedBytes == null || delayedBytes == ByteString.EMPTY);
    }

    /**
     * Clears the value state of this instance.
     *
     * <p>LazyField is not thread-safe for write access. Synchronizations are needed under read/write
     * situations.
     */
    public void clear() {
        // Don't clear the ExtensionRegistry. It might prove useful later on when merging in another
        // value, but there is no guarantee that it will contain all extensions that were directly set
        // on the values that need to be merged.
        delayedBytes = null;
        value = null;
        memoizedBytes = null;
    }

    /**
     * Overrides the contents of this LazyField.
     *
     * <p>LazyField is not thread-safe for write access. Synchronizations are needed under read/write
     * situations.
     */
    public void set(LazyFieldLite other) {
        this.delayedBytes = other.delayedBytes;
        this.value = other.value;
        this.memoizedBytes = other.memoizedBytes;
        // If the other LazyFieldLite was created by directly setting the value rather than first by
        // parsing, then it will not have an extensionRegistry. In this case we hold on to the existing
        // extensionRegistry, which has no guarantees that it has all the extensions that will be
        // directly set on the value.
        if (other.extensionRegistry != null) {
            this.extensionRegistry = other.extensionRegistry;
        }
    }

    /**
     * Returns message instance. It may do some thread-safe delayed parsing of bytes.
     *
     * @param defaultInstance its message's default instance. It's also used to get parser for the
     *                        message type.
     */
    public MessageLite getValue(MessageLite defaultInstance) {
        ensureInitialized(defaultInstance);
        return value;
    }

    /**
     * Sets the value of the instance and returns the old value without delay parsing anything.
     *
     * <p>LazyField is not thread-safe for write access. Synchronizations are needed under read/write
     * situations.
     */
    public MessageLite setValue(MessageLite value) {
        MessageLite originalValue = this.value;
        this.delayedBytes = null;
        this.memoizedBytes = null;
        this.value = value;
        return originalValue;
    }

    /**
     * Merges another instance's contents. In some cases may drop some extensions if both fields
     * contain data. If the other field has an {@code ExtensionRegistry} but this does not, then this
     * field will copy over that {@code ExtensionRegistry}.
     *
     * <p>LazyField is not thread-safe for write access. Synchronizations are needed under read/write
     * situations.
     */
    public void merge(LazyFieldLite other) {
        if (other.containsDefaultInstance()) {
            return;
        }

        if (this.containsDefaultInstance()) {
            set(other);
            return;
        }

        // If the other field has an extension registry but this does not, copy over the other extension
        // registry.
        if (this.extensionRegistry == null) {
            this.extensionRegistry = other.extensionRegistry;
        }

        // In the case that both of them are not parsed we simply concatenate the bytes to save time. In
        // the (probably rare) case that they have different extension registries there is a chance that
        // some of the extensions may be dropped, but the tradeoff of making this operation fast seems
        // to outway the benefits of combining the extension registries, which is not normally done for
        // lite protos anyways.
        if (this.delayedBytes != null && other.delayedBytes != null) {
            this.delayedBytes = this.delayedBytes.concat(other.delayedBytes);
            return;
        }

        // At least one is parsed and both contain data. We won't drop any extensions here directly, but
        // in the case that the extension registries are not the same then we might in the future if we
        // need to serialize and parse a message again.
        if (this.value == null && other.value != null) {
            setValue(mergeValueAndBytes(other.value, this.delayedBytes, this.extensionRegistry));
            return;
        } else if (this.value != null && other.value == null) {
            setValue(mergeValueAndBytes(this.value, other.delayedBytes, other.extensionRegistry));
            return;
        }

        // At this point we have two fully parsed messages.
        setValue(this.value.toBuilder().mergeFrom(other.value).build());
    }

    /**
     * Merges another instance's contents from a stream.
     *
     * <p>LazyField is not thread-safe for write access. Synchronizations are needed under read/write
     * situations.
     */
    public void mergeFrom(CodedInputStream input, ExtensionRegistryLite extensionRegistry)
            throws IOException {
        if (this.containsDefaultInstance()) {
            setByteString(input.readBytes(), extensionRegistry);
            return;
        }

        // If the other field has an extension registry but this does not, copy over the other extension
        // registry.
        if (this.extensionRegistry == null) {
            this.extensionRegistry = extensionRegistry;
        }

        // In the case that both of them are not parsed we simply concatenate the bytes to save time. In
        // the (probably rare) case that they have different extension registries there is a chance that
        // some of the extensions may be dropped, but the tradeoff of making this operation fast seems
        // to outway the benefits of combining the extension registries, which is not normally done for
        // lite protos anyways.
        if (this.delayedBytes != null) {
            setByteString(this.delayedBytes.concat(input.readBytes()), this.extensionRegistry);
            return;
        }

        // We are parsed and both contain data. We won't drop any extensions here directly, but in the
        // case that the extension registries are not the same then we might in the future if we
        // need to serialize and parse a message again.
        try {
            setValue(value.toBuilder().mergeFrom(input, extensionRegistry).build());
        } catch (InvalidProtocolBufferException e) {
            // Nothing is logged and no exceptions are thrown. Clients will be unaware that a proto
            // was invalid.
        }
    }

    private static MessageLite mergeValueAndBytes(
            MessageLite value, ByteString otherBytes, ExtensionRegistryLite extensionRegistry) {
        try {
            return value.toBuilder().mergeFrom(otherBytes, extensionRegistry).build();
        } catch (InvalidProtocolBufferException e) {
            // Nothing is logged and no exceptions are thrown. Clients will be unaware that a proto
            // was invalid.
            return value;
        }
    }

    /**
     * Sets this field with bytes to delay-parse.
     */
    public void setByteString(ByteString bytes, ExtensionRegistryLite extensionRegistry) {
        checkArguments(extensionRegistry, bytes);
        this.delayedBytes = bytes;
        this.extensionRegistry = extensionRegistry;
        this.value = null;
        this.memoizedBytes = null;
    }

    /**
     * Due to the optional field can be duplicated at the end of serialized bytes, which will make the
     * serialized size changed after LazyField parsed. Be careful when using this method.
     */
    public int getSerializedSize() {
        // We *must* return delayed bytes size if it was ever set because the dependent messages may
        // have memoized serialized size based off of it.
        if (memoizedBytes != null) {
            return memoizedBytes.size();
        } else if (delayedBytes != null) {
            return delayedBytes.size();
        } else if (value != null) {
            return value.getSerializedSize();
        } else {
            return 0;
        }
    }

    /**
     * Returns a BytesString for this field in a thread-safe way.
     */
    public ByteString toByteString() {
        if (memoizedBytes != null) {
            return memoizedBytes;
        }
        // We *must* return delayed bytes if it was set because the dependent messages may have
        // memoized serialized size based off of it.
        if (delayedBytes != null) {
            return delayedBytes;
        }
        synchronized (this) {
            if (memoizedBytes != null) {
                return memoizedBytes;
            }
            if (value == null) {
                memoizedBytes = ByteString.EMPTY;
            } else {
                memoizedBytes = value.toByteString();
            }
            return memoizedBytes;
        }
    }

    /**
     * Writes this lazy field into a {@link Writer}.
     */
    void writeTo(Writer writer, int fieldNumber) throws IOException {
        if (memoizedBytes != null) {
            writer.writeBytes(fieldNumber, memoizedBytes);
        } else if (delayedBytes != null) {
            writer.writeBytes(fieldNumber, delayedBytes);
        } else if (value != null) {
            writer.writeMessage(fieldNumber, value);
        } else {
            writer.writeBytes(fieldNumber, ByteString.EMPTY);
        }
    }

    /**
     * Might lazily parse the bytes that were previously passed in. Is thread-safe.
     */
    protected void ensureInitialized(MessageLite defaultInstance) {
        if (value != null) {
            return;
        }
        synchronized (this) {
            if (value != null) {
                return;
            }
            try {
                if (delayedBytes != null) {
                    // The extensionRegistry shouldn't be null here since we have delayedBytes.
                    MessageLite parsedValue =
                            defaultInstance.getParserForType().parseFrom(delayedBytes, extensionRegistry);
                    this.value = parsedValue;
                    this.memoizedBytes = delayedBytes;
                } else {
                    this.value = defaultInstance;
                    this.memoizedBytes = ByteString.EMPTY;
                }
            } catch (InvalidProtocolBufferException e) {
                // Nothing is logged and no exceptions are thrown. Clients will be unaware that this proto
                // was invalid.
                this.value = defaultInstance;
                this.memoizedBytes = ByteString.EMPTY;
            }
        }
    }

    private static void checkArguments(ExtensionRegistryLite extensionRegistry, ByteString bytes) {
        if (extensionRegistry == null) {
            throw new NullPointerException("found null ExtensionRegistry");
        }
        if (bytes == null) {
            throw new NullPointerException("found null ByteString");
        }
    }
}
